The field of glass-ceramics is founded in U.S. Pat. No. 2,920,971. As is explained therein, glass-ceramic articles are produced through the controlled crystallization of precursor glass articles. The preparation of glass-ceramics typically involves three general steps: first, a glass-forming batch, often containing a nucleating agent, is melted; second, that melt is simultaneously cooled to a temperature at least within and commonly below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, third, that glass body is exposed to a temperature above the transformation range to cause the glass to crystallize in situ. Frequently, crystallization will be accomplished in two stages; viz., the glass body will initially be heated to a temperature somewhat above the transformation range to induce the generation of nuclei therein, after which the temperature is raised, often above the softening point of the glass, to effect the growth of crystals on those nuclei. Normally, the melted batch will be cooled sufficiently rapidly that devitrification of the glass is avoided. The transformation range has been defined as the temperature at which a molten mass becomes an amorphous solid, that temperature typically being deemed to reside in the vicinity of the annealing point of a glass.
The crystallization developed in a glass-ceramic is usually uniformly fine-grained, homogeneously dispersed throughout the body, and comprises a substantial proportion, often greater than 50% by volume, of the body. The residual glassy matrix therein will be a composition different from that of the parent glass since the components comprising the crystals have been removed therefrom. Glass-ceramic bodies are normally non-porous and free from voids. Because a glass-ceramic is prepared from a precursor glass body, the conventional glass forming methods of production such as blowing, casting, drawing, pressing, rolling, and spinning can be utilized.